The invention is directed to a solid state electronic relay.
An example of an existing solid state relay is shown in FIG. 3. This solid state relay has two input terminals, IN.sub.1 and IN.sub.2, and two output terminals, OUT.sub.1, and OUT.sub.2. Between the input and output terminals are a thyristor THY, two photothyristor couplers, PHT.sub.1 and PHT.sub.2, a snubber circuit K and a surge arrester SA.
In operation, an input signal is applied to input terminals IN.sub.1 and IN.sub.2 to control the AC output. AC circuit Z, consisting of AC power supply and load L, is connected to output terminals OUT.sub.1 and OUT.sub.2. In this example, the input signal applied to input terminals IN.sub.1, and IN.sub.2 is DC, however, it is also possible to use AC input. The photothyristor couplers PHT.sub.1 and PHT.sub.2 drive thyristor THY in response to the input signal applied to input terminals IN.sub.1 and IN.sub.2. In turn, the thyristor THY controls the current flow in AC circuit Z.
Photothyristor couplers PHT.sub.1 and PHT.sub.2 can both can be turned ON only when the voltage from power supply AC approaches the zero voltage cross-point. The photothyristor couplers have what is known as a zero-crossing function. Each of the photothyristor couplers PHT.sub.1 and PHT.sub.2 has a built-in zero-crossing detector circuit (not shown), a light emitting diode D.sub.1, or D.sub.2 as a luminous element, and a photothyristor TR.sub.1, or TR.sub.2. Light emitting diodes D.sub.1 and D.sub.2 are connected in series to input terminals IN.sub.1 and IN.sub.2 ; photothyristors TR.sub.1 and TR.sub.2 are connected separately in series to output terminals OUT.sub.1 and OUT.sub.2.
Snubber circuit K prevents the accidental operation of thyristor THY which might be induced by back current flow when load L is inductive. In this example, the snubber circuit has two resistors, R.sub.1 and R.sub.2, and two capacitors, C.sub.1 and C.sub.2 connected in series.
A surge arrester SA, which in this example is a varistor, protects thyristor THY and photothyristors TR.sub.1 and TR.sub.2 in case an overvoltage is generated by AC circuit Z.
R.sub.0 is a resistor which sets the gate current for thyristor THY.
When no DC input signal is applied to input terminals IN.sub.1 and IN.sub.2 the light emitting diodes D.sub.1 and D.sub.2 do not emit light. Thus, photothyristors TR.sub.1 and TR.sub.2 are not conductive, and thyristor THY is off. As such, power is not supplied from power supply AC to load L.
In this state, the voltage from power supply AC is divided between the two photothyristors TR.sub.1 and TR.sub.2 which are in series. For example, if the voltage of power supply AC is 400 V rms, a voltage of 200 V rms (peak voltage of 282 V) is applied to each of photothyristors TR.sub.1 and TR.sub.2.
When a DC input signal is applied to input terminals IN.sub.1 and IN.sub.2, light emitting diodes D.sub.1 and D.sub.2 in photothyristor couplers PHT.sub.1 and PHT.sub.2 emit light. Referring to FIG. 4, if the input signal is applied at a point in time when the waveform is not within the region delta V (FIG. 4(b)) of low voltage near the zero-crossing point of the power supply voltage (at time t.sub.1 in FIG. 4(b), for example), photothyristors TR.sub.1 and TR.sub.2 do not turn ON, so thyristor THY does not go ON and power is not supplied from power supply AC to load L.
When the input signal is applied at a point in time when the waveform is within the region delta V of low voltage near the zero-crossing point of the power supply voltage (at time t.sub.2 for example), photothyristors TR.sub.1 and TR.sub.2 will turn ON. Thyristor THY will then turn ON and the power is supplied from power supply AC to load L.
Photothyristors TR.sub.1 and TR.sub.2 only turn ON when the power supply voltage is in the low-voltage region delta V near the zero crossing point. Thus, a high voltage is never applied to photothyristors TR.sub.1 and TR.sub.2.
When the input signal applied to input terminals IN.sub.1 and IN.sub.2 is turned OFF and the power supply voltage falls below the holding current of photothyristors TR.sub.1 and TR.sub.2 (as at time t.sub.3), elements TR.sub.1 and TR.sub.2 are no longer conductive. Thyristor THY is also no longer conductive, and so the flow of electricity from power supply to load L is cut off.
The conventional solid state relay described above employs photothyristor couplers PHT.sub.1 and PHT.sub.2 that have zero-crossing functions. Because elements TR.sub.1 and TR.sub.2 vary somewhat in manufacture, their turn on time is necessarily slightly out of phase. However, because of the zero-crossing function, there is no possibility that elements TR.sub.1 and TR.sub.2 will be damaged by excessive power from power supply AC.
Photothyristors TR.sub.1 and TR.sub.2 are protected by the fact that when they are not conductive, the power supply voltage is divided between them, and when they are conductive, the power supply voltage is always near the zero crossing point. The maximum value of the power supply voltage is, therefore, never applied to either photothyristor TR.sub.1 or TR.sub.2.
However, in a conventional relay, such as the one shown in FIG. 3, the phase angle at which the AC power supply switches ON and OFF cannot be controlled, due to the zerocrossing function.
This poses a problem, since there are a number of devices in which it is necessary to control the phase angle at which AC power is applied, such as motor controllers or dimmers. As discussed above, in a relay that employs photothyristor couplers PHT.sub.1 and PHT.sub.2 with zero-crossing functions (FIG. 3), when an input signal is applied to input terminals IN.sub.1 and IN.sub.2 the output terminals OUT.sub.1 and OUT.sub.2 do not immediately conduct, but rather conduct depending on the position of the supply voltage waveform.
It would be conceivable to address this problem by substituting elements without a zero-crossing function for photothyristor couplers PHT.sub.1 and PHT.sub.2 in the solid state relay shown in FIG. 3. If this were done, output terminals OUT.sub.1 and OUT.sub.2 would immediately conduct when an input signal is applied to input terminals IN.sub.1 and IN.sub.2, regardless of the phase angle of the AC waveform. In this way the phase of switching could be controlled.
However, if photothyristor couplers PHT .sub.1 and PHT.sub.2 are used which completely lack a zero-crossing function, the following problem arises. Assuming that the voltages at both terminals of photothyristors TR.sub.1 and TR.sub.2 are V.sub.1 and V.sub.2 respectively, and that photothyristors TR.sub.1 and TR.sub.2 are both OFF, as shown in FIG. 5 (the period labeled T.sub.a) the voltage from the AC power supply is divided between photothyristors TR.sub.1 and TR.sub.2, and so is not applied in full to either of the photothyristors. Even if the supply voltage is at its maximum value V.sub.max, the terminal voltage on either photothyristor will be V.sub.1 =V.sub.2 =V.sub.max /2.
One of photothyristor couplers PHT.sub.1, and PHT.sub.2 will, in general, have a time lag, delta T, in its turn on time due to variations in its performance characteristics caused by the manufacturing process, as mentioned above. If thyristor TR.sub.1 turns ON slightly ahead of photothyristor TR.sub.2 and the supply voltage is in the vicinity of its maximum value, then the maximum voltage V.sub.max will be applied momentarily to photothyristor TR.sub.2 alone. The voltage across the photothyristor TR.sub.2, V.sub.2, will then go to V.sub.max, and element TR.sub.2 will be damaged.
To prevent photothyristors TR.sub.1 and TR.sub.2 from being damaged in this way, elements with breakdown voltages sufficiently higher than the maximum value V.sub.max of the power supply voltage are needed.
For example, if the voltage of the AC power supply is 400 V rms, zero-crossing function photothyristor couplers PHT.sub.1 and PHT.sub.2 each with a breakdown voltage of 600 V could be used. If photothyristor couplers PHT.sub.1 and PHT.sub.2 without a zero-crossing function are used, however, they would each need to have a breakdown voltage of 1200 V, which would drive up the cost of the relay.